Ion mobility separator with variable effective length

ABSTRACT

An ion mobility separator or spectrometer is disclosed comprising an inner cylinder and an outer cylinder defining an annular volume through which ions are transmitted. Spiral electrodes a-f are arranged on a surface of the inner cylinder and/or on a surface of the outer cylinder. A first device is arranged and adapted to maintain a DC electric field or a pseudo-potential force which acts to urge ions from a first end of the ion mobility separator or spectrometer to a second end of the ion mobility separator or spectrometer. A second device is arranged and adapted to apply one or more transient DC voltages to the spiral electrodes in order to urge ions towards the first end of the ion mobility separator or spectrometer. The net effect is to extend the effective path length of the ion mobility separator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/GB2012/050545, filed 13 Mar. 2012, which claims priority from andthe benefit of U.S. Provisional Patent Application Ser. No. 61/476,850filed on 19 Apr. 2011 and United Kingdom Patent Application No.1104238.9 filed on 14 Mar. 2011. The entire contents of theseapplications are incorporated herein by reference.

BACKGROUND TO THE INVENTION

The present invention relates to the field of mass spectrometry and inparticular ion mobility spectrometry.

Ion Mobility Spectrometry (“IMS”) is a well established analyticaltechnique where ionic species are separated according to their ionmobility by subjecting the ions to a weak electric field in the presenceof a buffer gas. A known ion mobility spectrometer comprises a lineartube filled with gas. A static homogeneous axial electric field ismaintained along the length of the tube. Ions experience an axial forcein one direction due to the electric field and an effective force in theother direction due to collisions with the buffer gas.

The ion mobility resolution R of such a system can be expressed by Eqn.1:

$\begin{matrix}{R = {\frac{T}{\Delta\; t} = {\frac{K}{\Delta\; K} = \sqrt{\frac{LEq}{16\;{kT}\;{\ln(2)}}}}}} & (1)\end{matrix}$wherein L is the length of the tube (m), E is the electric field (V/m),K is Boltzmann's constant, T is the temperature of the buffer gas (K), Kis the mobility (M²V⁻¹s⁻¹) and q is the charge on the ion.

To improve ion mobility resolution R the length of the drift tube may beincreased or the electric field may be increased. However, therelationship in Eqn. 1 only holds approximately below a low electricfield limit wherein the ratio of electric field to buffer gas numberdensity is below a certain value. To allow the field to be increasedwithout exceeding this value requires the pressure to be increased bythe same factor. Both these approaches lead to practical limitations inthe IMS resolution which can ultimately be achieved.

Another approach to increasing IMS resolution without increasing pathlength is described in Novel Ion Mobility Setup Combined with CollisionCell and Time of Flight Mass Spectrometer, J. Am Soc Mass Spectrom,2006, Volume 17, Issue 5, p 691-699, Alexander Loboda. In this methodthe buffer gas is allowed to flow in a direction opposing the electricfield. The combination of gas flow and DC field allow ions to remainlonger in the cell thereby experiencing more collisions with the buffergas. This results in marked improvements in mobility resolution withoutincreasing the physical length of the mobility device. Careful design ofthe gas flow dynamics of the IMS cell must be considered in thisapproach to avoid turbulent flow effects which will cause a degrading ofIMS resolution.

It is desired to provide an improved mass spectrometer and method ofmass spectrometry.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is providedapparatus for separating ions according to one or more physico-chemicalproperties, wherein the apparatus is arranged and adapted to create anion channel in which ions are confined in use and wherein the ions arecaused to separate according to the one or more physico-chemicalproperties along an axis of the ion channel or along the ion channeltowards a first end and wherein the apparatus is further arranged andadapted to move the axis of the ion channel or the ion channel away fromthe first end.

According to the preferred embodiment the axis of the ion channel and/orthe ion channel is non-linear. For example, the axis of the ion channeland/or the ion channel may be helical, spiral or curved.

According to a less preferred embodiment the axis of the ion channeland/or the ion channel may be linear.

According to the preferred embodiment the first end comprises an exitand is opposed to a second end through which ions initially entered theapparatus i.e. ions travel from an ion entrance to an ion exit throughthe apparatus. However, according to a less preferred embodiment thefirst end may comprise an entrance through which ions initially enteredthe apparatus. According to this embodiment the direction in which ionscome out of or exit the apparatus can be reversed by moving the ionchannel faster than the velocity of a particular ion.

The effect of moving the axis of the ion channel or the ion channel awayfrom the exit is to increase the effective path length along which ionsseparate according to the one or more physico-chemical properties.

The ion channel preferably comprises a DC potential well.

The ion channel is preferably formed between: (i) a first DC voltagegradient, a first DC potential, a first electrostatic barrier, a firstDC potential barrier or a first pseudo-potential; and (ii) a secondmoving DC potential barrier, a second moving electrostatic barrier, asecond moving DC potential barrier or a second moving pseudo-potentialbarrier.

Ions are preferably confined within the ion channel in a first plane ordirection by DC voltages, DC potentials or electrostatic potentials.

Ions are preferably confined within the ion channel in a second plane ordirection by RF voltages, RF potentials or pseudo-potentials, whereinthe second plane or direction is substantially orthogonal to the firstplane or direction.

The apparatus preferably comprises:

a first device arranged and adapted to cause ions to separate accordingto the one or more physico-chemical properties in a first direction oralong the axis of the ion channel or along the ion channel with avelocity which is substantially dependent upon the one or morephysico-chemical properties.

The first device is preferably arranged and adapted to apply or maintaina first electrostatic potential or force, a first DC potential or force,or a first pseudo-potential or force along at least a portion of theapparatus or along the axis of the ion channel or along the ion channelin order to cause ions to separate according to the one or morephysico-chemical properties.

The first device is preferably arranged and adapted to apply or maintaina first electrostatic potential or force, a first DC potential or force,or a first pseudo-potential or force along at least a portion of theapparatus or along the axis of the ion channel or along the ion channelin order to cause ions to separate according to the one or morephysico-chemical properties.

The first device is preferably arranged and adapted:

(i) to maintain a first DC voltage gradient, a first DC potential or afirst DC electric field across at least a portion or at least 10%, 20%,30%, 40%, 50%, 60%, 70%, 80% or 90% of the axial length of the apparatusor along the axis of the ion channel or along the ion channel; and/or

(ii) to apply a first electrostatic potential or force along at least aportion or at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of theaxial length of the apparatus or along the axis of the ion channel oralong the ion channel; and/or

(iii) to apply one or more first transient DC voltages or potentials toa plurality of electrodes in order to urge ions along at least a portionor at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of the axiallength of the apparatus or along the axis of the ion channel or alongthe ion channel; and/or

(iv) to apply a first RF voltage comprising three or more phases to aplurality of electrodes, wherein different electrodes are connected todifferent phases of the RF voltage and wherein the first RF voltageurges ions along at least a portion or at least 10%, 20%, 30%, 40%, 50%,60%, 70%, 80% or 90% of the axial length of the apparatus or along theaxis of the ion channel or along the ion channel; and/or

(v) to apply a first pseudo-potential or force wherein the amplitudeand/or frequency of an RF voltage applied to a plurality of electrodesvaries, increases or decreases along at least a portion or at least 10%,20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of the axial length of theapparatus or along the axis of the ion channel or along the ion channel.

The apparatus preferably further comprises a second device arranged andadapted to drive the ions in a second direction with a velocity which issubstantially independent of the one or more physico-chemicalproperties.

The second device is preferably arranged and adapted to apply ormaintain a second moving electrostatic potential barrier, a secondmoving DC potential barrier or a second moving pseudo-potential barrieralong at least a portion of the apparatus in order to drive the ions inthe second direction.

The first electrostatic potential or force, the first DC potential orforce, or the first pseudo-potential or force is preferablysubstantially greater than, equal to, or less than the second movingelectrostatic potential barrier, the second moving DC potential barrieror the second moving pseudo-potential barrier.

The second device is preferably arranged and adapted:

(i) to apply a second moving electrostatic potential barrier along atleast a portion or at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or90% of the axial length of the apparatus; and/or

(ii) to apply one or more second transient DC voltages or potentials toa plurality of electrodes in order to urge ions along at least a portionor at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of the axiallength of the apparatus; and/or

(iii) to apply a second RF voltage comprising three or more phases to aplurality of electrodes, wherein different electrodes are connected todifferent phases of the RF voltage and wherein the second RF voltageurges ions along at least a portion or at least 10%, 20%, 30%, 40%, 50%,60%, 70%, 80% or 90% of the axial length of the apparatus; and/or

(iv) to apply a second moving pseudo-potential barrier wherein theamplitude and/or frequency of an RF voltage applied to a plurality ofelectrodes varies, increases or decreases along at least a portion or atleast 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of the axial lengthof the apparatus.

According to an embodiment:

(i) the first electrostatic potential or force is substantially greaterthan, equal to, or less than the second moving electrostatic potentialbarrier; and/or

(ii) the first DC potential or the first DC electric field issubstantially greater than, equal to, or less than the second moving DCpotential barrier;

(iii) the first transient DC voltages or potentials have an amplitudewhich is substantially greater than, equal to or less than the secondtransient DC voltages or potentials; and/or

(iv) the first RF voltage has an amplitude which is substantiallygreater than, equal to or less than the second RF voltage; and/or

(v) the first pseudo-potential or force is substantially greater than,equal to, or less than the second pseudo-potential barrier.

The first device is preferably arranged and adapted to drive or urgeions in the first direction with a first velocity or a first componentof velocity and the second device is preferably arranged and adapted todrive or urge ions in the second direction with a second velocity orsecond component of velocity, wherein the first velocity or firstcomponent of velocity is substantially greater than, equal to or lessthan the second velocity or the second component of velocity.

The first electrostatic potential or force, the first DC potential orforce, or the first pseudo-potential or force preferably has a non-zerocomponent

(i) in a direction which is substantially parallel to the second movingelectrostatic potential barrier, the second moving DC potential barrier,or the second moving pseudo-potential barrier at a particular instancein time; and/or

(ii) in a direction substantially orthogonal to a direction of movementof the second moving electrostatic potential barrier, the second movingDC potential barrier, or the second moving pseudo-potential barrier.

According to an embodiment the first device drives ions with a firstvelocity or force in the first direction and the second device drivesions with a second velocity or force in the second direction, whereineither:

(i) the first direction is inclined to or offset relative to the seconddirection; and/or

(ii) the first direction is not co-directional with the seconddirection; and/or

(iii) the first direction is not counter to the second direction; and/or

(iv) the first velocity or force has a non-zero component of velocity orforce in a direction substantially orthogonal to the second direction.

The first device preferably drives ions with a first velocity or forceand the second device drives ions with a second velocity or force,wherein either the first velocity or force and/or the second velocity orforce varies with time and/or position.

According to the preferred embodiment the physico-chemical propertycomprises ion mobility. The apparatus preferably comprises an ionmobility spectrometer or separator.

According to a less preferred embodiment the physico-chemical propertycomprises differential ion mobility. According to a less preferredembodiment the apparatus comprises a differential ion mobilityspectrometer or a Field Asymmetric Ion Mobility Spectrometer (“FAIMS”)device.

According to a less preferred embodiment the physico-chemical propertycomprises mass or mass to charge ratio. The apparatus may comprise amass or mass to charge ratio analyser.

According to an embodiment the apparatus comprises:

an inner cylinder and an outer cylinder, wherein the inner cylinder andthe outer cylinder define an annular volume through which ions aretransmitted in use; and

wherein one or more spiral or helical electrodes are arranged on asurface of the inner cylinder and/or on a surface of the outer cylinder.

A first device is preferably arranged and adapted to maintain a DCelectric field and/or a pseudo-potential force which acts to urge ionsfrom a first end of the apparatus to a second end of the apparatus; and

a second device is preferably arranged and adapted to apply one or moretransient DC voltages to the one or more spiral or helical electrodes inorder to urge ions towards the first end of the apparatus.

The inner cylinder and/or the outer cylinder preferably have a circular,elliptical, non-circular, rectangular or irregular cross section.

The pitch of the one or more spiral or helical electrodes is preferablyconstant, increases, decreases or varies along the length of theapparatus.

According to an embodiment the apparatus further comprises a device forapplying an RF voltage to the one or more spiral or helical electrodesin order to confine ions radially within the annular volume.

The apparatus preferably further comprises a device arranged and adaptedto supply a buffer gas to the annular volume between the outer cylinderand the inner cylinder.

The inner cylinder and/or the outer cylinder may comprise anon-conductive or dielectric material.

The apparatus may further comprise one or more auxiliary electrodesprovided on an opposite side of the inner cylinder and/or the outercylinder to that of the one or more spiral or helical electrodes.

An RF voltage may be applied to the one or more auxiliary electrodes.

In a mode of operation ions are preferably urged along a spiral orhelical path through the apparatus.

Ions are preferably arranged to enter the apparatus via the annularvolume and/or via an aperture in the inner cylinder and/or via anaperture in the outer cylinder.

Ions are preferably separated temporally according to their ion mobilityor according to their rate of change of ion mobility with electric fieldstrength.

According to an embodiment:

the apparatus further comprises a plurality of segmented planarelectrodes; and

a second device arranged and adapted to apply DC voltages to thesegmented planar electrodes so that one or more diagonal or inclined DCvoltage barriers are translated along at least a portion of the lengthof the apparatus.

According to an embodiment:

the apparatus further comprises a plurality of inner ring electrodes anda plurality of outer ring electrodes, wherein the inner ring electrodesand the outer ring electrodes define an annular volume through whichions are transmitted in use, wherein the plurality of inner ringelectrodes and/or the plurality of outer ring electrodes are radiallysegmented into a plurality of segmented electrodes.

The first device and the second device are preferably operated atsubstantially the same time so that the net effect is to extend the pathlength of ions passing through the apparatus.

According to an aspect of the present invention there is provided a massspectrometer comprising apparatus as described above.

According to an aspect of the present invention there is provided amethod of separating ions according to one or more physico-chemicalproperties, comprising:

creating an ion channel in which ions are confined and causing the ionsto separate according to the one or more physico-chemical propertiesalong an axis of the ion channel or along the ion channel towards afirst end; and

moving the axis of the ion channel or the ion channel away from thefirst end.

According to an aspect of the present invention there is providedapparatus for separating ions according to one or more physico-chemicalproperties comprising:

a plurality of electrodes;

a first device arranged and adapted to cause ions to separate accordingto the one or more physico-chemical properties in a first direction witha velocity which is substantially dependent upon the one or morephysico-chemical properties; and

a second device arranged and adapted to drive the ions in a seconddirection with a velocity which is substantially independent of the oneor more physico-chemical properties.

According to an aspect of the present invention there is provided amethod of separating ions according to one or more physico-chemicalproperties comprising:

causing ions to separate according to the one or more physico-chemicalproperties in a first direction with a velocity which is substantiallydependent upon the one or more physico-chemical properties; and

driving the ions in a second direction with a velocity which issubstantially independent of the one or more physico-chemicalproperties.

Ions are preferably caused to separate according to the one or morephysico-chemical properties in the first direction at substantially thesame time that they are driven in the second direction so that the neteffect is to extend the path length of ions.

According to an aspect of the present invention there is provided an ionmobility separator comprising:

a first device arranged and adapted to apply or maintain a firstelectrostatic potential or field, a first DC potential or field, or afirst pseudo-potential or field in a first direction so as to cause ionsto separate according to their ion mobility in the first direction; and

a second device arranged and adapted to drive the ions in a seconddirection by driving the ions with a second moving electrostaticpotential barrier, a second moving DC potential barrier, or a secondmoving pseudo-potential barrier;

wherein ions are separated according to their ion mobility: (i) in adirection substantially parallel to the second moving electrostaticpotential barrier, the second moving DC potential barrier, or the secondmoving pseudo-potential barrier at a particular instance in time; and/or(ii) in a direction substantially orthogonal to a direction of movementof the second moving electrostatic potential barrier, the second movingDC potential barrier, or the second moving pseudo-potential barrier.

The first device is preferably arranged and adapted to drive or urge theions in the first direction with a first component of velocity and thesecond device is preferably arranged and adapted to drive or urge ionsin the second direction with a second component of velocity, wherein thefirst component of velocity is substantially greater than the secondcomponent of velocity.

The first electrostatic potential or force, the first DC potential orforce, or the first pseudo-potential or force preferably has a non-zerocomponent:

(i) in a direction which is substantially parallel to the second movingelectrostatic potential barrier, the second moving DC potential barrier,or the second moving pseudo-potential barrier at a particular instancein time; and/or

(ii) in a direction substantially orthogonal to a direction of movementof the second moving electrostatic potential barrier, the second movingDC potential barrier, or the second moving pseudo-potential barrier.

According to an aspect of the present invention there is provided amethod of separating ions according to their ion mobility comprising:

applying or maintaining a first electrostatic potential or field, afirst DC potential or field, or a first pseudo-potential or field in afirst direction so as to cause ions to separate according to their ionmobility in the first direction; and

driving the ions in a second direction by driving the ions with a secondmoving electrostatic potential barrier, a second moving DC potentialbarrier, or a second moving pseudo-potential barrier;

wherein ions are separated according to their ion mobility: (i) in adirection substantially parallel to the second moving electrostaticpotential barrier, the second moving DC potential barrier, or the secondmoving pseudo-potential barrier at a particular instance in time; and/or(ii) in a direction substantially orthogonal to a direction of movementof the second moving electrostatic potential barrier, the second movingDC potential barrier, or the second moving pseudo-potential barrier.

The method preferably further comprises driving or urging the ions inthe first direction with a first component of velocity andsimultaneously driving or urging the ions in the second direction with asecond component of velocity, wherein the first component of velocity issubstantially greater than the second component of velocity.

The first electrostatic potential or force, the first DC potential orforce, or the first pseudo-potential or force preferably has a non-zerocomponent

(i) in a direction which is substantially parallel to the second movingelectrostatic potential barrier, the second moving DC potential barrier,or the second moving pseudo-potential barrier at a particular instancein time; and/or

(ii) in a direction substantially orthogonal to a direction of movementof the second moving electrostatic potential barrier, the second movingDC potential barrier, or the second moving pseudo-potential barrier.

According to an aspect of the present invention there is provided an ionmobility separator for separating ions according to their ion mobilitycomprising:

a device arranged and adapted to create a DC ion channel in which ionsare confined by DC potentials and wherein the ions are caused toseparate according to their ion mobility along an axis of the ionchannel or along the ion channel towards an exit of the ion mobilityseparator; and

a device arranged and adapted to move the axis of the ion channel or theion channel away from the exit thereby increasing the effective pathlength of the ion mobility separator.

According to an aspect of the present invention there is provided amethod of separating ions according to their ion mobility comprising:

creating a DC ion channel in which ions are confined by DC potentialsand wherein the ions are caused to separate according to their ionmobility along an axis of the ion channel or along the ion channeltowards an exit of the ion mobility separator; and

moving the axis of the ion channel or the ion channel away from the exitthereby increasing the effective path length of the ion mobilityseparator.

According to an aspect of the present invention there is provided an ionmobility separator or spectrometer comprising:

an inner cylinder and an outer cylinder, wherein the inner cylinder andthe outer cylinder define an annular volume through which ions aretransmitted in use;

one or more spiral electrodes arranged on a surface of the innercylinder and/or on a surface of the outer cylinder;

a first device arranged and adapted to maintain a DC electric fieldand/or a pseudo-potential force which acts to urge ions from a first endof the ion mobility separator or spectrometer to a second end of the ionmobility separator or spectrometer; and

a second device arranged and adapted to apply one or more transient DCvoltages to the one or more spiral electrodes in order to urge ionstowards the first end of the ion mobility separator or spectrometer.

According to an aspect of the present invention there is provided amethod of separating ions according to their ion mobility comprising:

providing an inner cylinder and an outer cylinder, wherein the innercylinder and the outer cylinder define an annular volume through whichions are transmitted in use;

providing one or more spiral electrodes arranged on a surface of theinner cylinder and/or on a surface of the outer cylinder;

maintaining a DC electric field which acts to urge ions from a first endof the ion mobility separator or spectrometer to a second end of the ionmobility separator or spectrometer; and

applying one or more transient DC voltages to the one or more spiralelectrodes in order to urge ions towards the first end of the ionmobility separator or spectrometer.

According to an aspect of the present invention there is provided amethod of separating ions comprising:

supplying ions to a radio frequency ion guide;

driving ions in one direction such that ions with differentcharacteristics separate; and

driving ions in the opposite direction in a manner effectivelyindependent of the ions characteristics, such that ions with differentcharacteristics do not effectively separate.

According to the preferred embodiment the driving forces areelectrostatic.

According to the preferred embodiment components of the two opposingdriving forces are preferably substantially orthogonal to each other.

According to the preferred embodiment at least one of the driving forcesis preferably time and or position varying.

According to the preferred embodiment the separation is preferablyrelated to ion mobility in a buffer gas.

According to another embodiment the separation may be related to themass to charge ratio of the ions.

According to an embodiment the mass spectrometer may further comprise:

(a) an ion source selected from the group consisting of: (i) anElectrospray ionisation (“ESI”) ion source; (ii) an Atmospheric PressurePhoto Ionisation (“APPI”) ion source; (iii) an Atmospheric PressureChemical Ionisation (“APCI”) ion source; (iv) a Matrix Assisted LaserDesorption Ionisation (“MALDI”) ion source; (v) a Laser DesorptionIonisation (“LDI”) ion source; (vi) an Atmospheric Pressure Ionisation(“API”) ion source; (vii) a Desorption Ionisation on Silicon (“DIOS”)ion source; (viii) an Electron Impact (“EI”) ion source; (ix) a ChemicalIonisation (“CI”) ion source; (x) a Field Ionisation (“FI”) ion source;(xi) a Field Desorption (“FD”) ion source; (xii) an Inductively CoupledPlasma (“ICP”) ion source; (xiii) a Fast Atom Bombardment (“FAB”) ionsource; (xiv) a Liquid Secondary Ion Mass Spectrometry (“LSIMS”) ionsource; (xv) a Desorption Electrospray Ionisation (“DESI”) ion source;(xvi) a Nickel-63 radioactive ion source: (xvii) an Atmospheric PressureMatrix Assisted Laser Desorption Ionisation ion source; (xviii) aThermospray ion source; (xix) an Atmospheric Sampling Glow DischargeIonisation (“ASGDI”) ion source; and (xx) a Glow Discharge (“GD”) ionsource; and/or

(b) one or more continuous or pulsed ion sources; and/or

(c) one or more ion guides; and/or

(d) one or more ion mobility separation devices and/or one or more FieldAsymmetric Ion Mobility Spectrometer devices; and/or

(e) one or more ion traps or one or more ion trapping regions; and/or

(f) one or more collision, fragmentation or reaction cells selected fromthe group consisting of: (i) a Collisional Induced Dissociation (“CID”)fragmentation device; (ii) a Surface Induced Dissociation (“SID”)fragmentation device; (iii) an Electron Transfer Dissociation (“ETD”)fragmentation device; (iv) an Electron Capture Dissociation (“ECD”)fragmentation device; (v) an Electron Collision or Impact Dissociationfragmentation device; (vi) a Photo Induced Dissociation (“PID”)fragmentation device; (vii) a Laser Induced Dissociation fragmentationdevice; (viii) an infrared radiation induced dissociation device; (ix)an ultraviolet radiation induced dissociation device; (x) anozzle-skimmer interface fragmentation device; (xi) an in-sourcefragmentation device; (xii) an in-source Collision Induced Dissociationfragmentation device; (xiii) a thermal or temperature sourcefragmentation device; (xiv) an electric field induced fragmentationdevice; (xv) a magnetic field induced fragmentation device; (xvi) anenzyme digestion or enzyme degradation fragmentation device; (xvii) anion-ion reaction fragmentation device; (xviii) an ion-molecule reactionfragmentation device; (xix) an ion-atom reaction fragmentation device;(xx) an ion-metastable ion reaction fragmentation device; (xxi) anion-metastable molecule reaction fragmentation device; (xxii) anion-metastable atom reaction fragmentation device; (xxiii) an ion-ionreaction device for reacting ions to form adduct or product ions; (xxiv)an ion-molecule reaction device for reacting ions to form adduct orproduct ions; (xxv) an ion-atom reaction device for reacting ions toform adduct or product ions; (xxvi) an ion-metastable ion reactiondevice for reacting ions to form adduct or product ions; (xxvii) anion-metastable molecule reaction device for reacting ions to form adductor product ions; (xxviii) an ion-metastable atom reaction device forreacting ions to form adduct or product ions; and (xxix) an ElectronIonisation Dissociation (“EID”) fragmentation device; and/or

(g) a mass analyser selected from the group consisting of: (i) aquadrupole mass analyser; (ii) a 2D or linear quadrupole mass analyser,(iii) a Paul or 3D quadrupole mass analyser; (iv) a Penning trap massanalyser; (v) an ion trap mass analyser; (vi) a magnetic sector massanalyser; (vii) ion Cyclotron Resonance (“ICR”) mass analyser; (viii) aFourier Transform Ion Cyclotron Resonance (“FTICR”) mass analyser (ix)an electrostatic or orbitrap mass analyser; (x) a Fourier Transformelectrostatic or orbitrap mass analyser; (xi) a Fourier Transform massanalyser; (xii) a Time of Flight mass analyser; (xiii) an orthogonalacceleration Time of Flight mass analyser; and (xiv) a linearacceleration Time of Flight mass analyser and/or

(h) one or more energy analysers or electrostatic energy analysers;and/or

(i) one or more ion detectors; and/or

(j) one or more mass filters selected from the group consisting of: (j)a quadrupole mass filter; (ii) a 2D or linear quadrupole ion trap; (iii)a Paul or 3D quadrupole ion trap; (iv) a Penning ion trap; (v) an iontrap; (vi) a magnetic sector mass filter, (vii) a Time of Flight massfilter; and (viii) a Wein filter; and/or

(k) a device or ion gate for pulsing ions; and/or

(l) a device for converting a substantially continuous ion beam into apulsed ion beam.

The mass spectrometer may further comprise either:

(i) a C-trap and an Orbitrap® mass analyser comprising an outerbarrel-like electrode and a coaxial inner spindle-like electrode,wherein in a first mode of operation ions are transmitted to the C-trapand are then injected into the Orbitrap® mass analyser and wherein in asecond mode of operation ions are transmitted to the C-trap and then toa collision cell or Electron Transfer Dissociation device wherein atleast some ions are fragmented into fragment ions, and wherein thefragment ions are then transmitted to the C-trap before being injectedinto the Orbitrap® mass analyser; and/or

(ii) a stacked ring ion guide comprising one or more electrodes eachhaving an aperture through which ions are transmitted in use and whereinthe spacing of the electrodes increases along the length of the ionpath, and wherein the apertures in the electrodes in an upstream sectionof the ion guide have a first diameter and wherein the apertures in theelectrodes in a downstream section of the ion guide have a seconddiameter which is smaller than the first diameter, and wherein oppositephases of an AC or RF voltage are applied, in use, to successiveelectrodes.

The ion mobility spectrometer according to the preferred embodiment maycomprise one or more electrodes each having an aperture through whichions are transmitted in use. One or more transient DC voltages orpotentials or one or more DC voltage or potential waveforms arepreferably applied to the electrodes comprising the ion mobilityspectrometer in order to urge ions along the length of the ion mobilityspectrometer.

According to the preferred embodiment the one or more transient DCvoltages or potentials or the one or more DC voltage or potentialwaveforms create: (i) a potential hill or barrier; (ii) a potentialwell; (iii) multiple potential hills or barriers; (iv) multiplepotential wells; (v) a combination of a potential hill or barrier and apotential well; or (vi) a combination of multiple potential hills orbarriers and multiple potential wells.

The one or more transient DC voltage or potential waveforms preferablycomprise a repeating waveform or square wave.

An RF voltage is preferably applied to the electrodes of the ionmobility spectrometer and preferably has an amplitude selected from thegroup consisting of: (i)<50 V peak to peak; (ii) 50-100 V peak to peak;(iii) 100-150 V peak to peak; (iv) 150-200 V peak to peak; (v) 200-250 Vpeak to peak; (vi) 250-300 V peak to peak; (vii) 300-350 V peak to peak;(viii) 350-400 V peak to peak; (ix) 400-450 V peak to peak; (x) 450-500V peak to peak; (xi) 500-550 V peak to peak; (xxii) 550-600 V peak topeak; (xxiii) 600-650 V peak to peak; (xxiv) 650-700 V peak to peak;(xxv) 700-750 V peak to peak; (xxvi) 750-800 V peak to peak; (xxvii)800-850 V peak to peak; (xxvii) 850-900 V peak to peak; (xxix) 900-950 Vpeak to peak; (xxx) 950-1000 V peak to peak; and (xxxi)>1000 V peak topeak.

The RF voltage preferably have a frequency selected from the groupconsisting of: (i)<100 kHz; (ii) 100-200 kHz; (iii) 200-300 kHz; (iv)300-400 kHz; (v) 400-500 kHz, (vi) 0.5-1.0 MHz; (vii) 1.0-1.5 MHz;(viii) 1.5-2.0 MHz; (ix) 2.0-2.5 MHz; (x) 2.5-3.0 MHz; (xi) 3.0-3.5 MHz;(xii) 3.5-4.0 MHz; (xiii) 4.0-4.5 MHz; (xiv) 4.5-5.0 MHz; (xv) 5.0-5.5MHz; (xvi) 5.5-6.0 MHz; (xvii) 6.0-6.5 MHz; (xviii) 6.5-7.0 MHz; (xix)7.0-7.5 MHz; (xx) 7.5-8.0 MHz; (xxi) 8.0-8.5 MHz; (xxi) 8.5-9.0 MHz;(xxiii) 9.0-9.5 MHz; (xxiv) 9.5-10.0 MHz; and (xxv)>10.0 MHz.

The ion mobility spectrometer is preferably maintained at a pressureselected from the group comprising: (i)>0.001 mbar (ii)>0.01 mbar;(iii)>0.1 mbar; (iv)>1 mbar, (v)>10 mbar; (vi)>100 mbar; (vii)0.001-0.01 mbar (viii) 0.01-0.1 mbar; (ix) 0.1-1 mbar; (x) 1-10 mbar,and (xi) 10-100 mbar.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be described, byway of example only, and with reference to the accompanying drawings inwhich:

FIG. 1 shows an ion mobility separator according to a preferredembodiment;

FIG. 2 shows a diagram of the preferred embodiment;

FIG. 3 shows a single helical DC barrier formed within the device;

FIG. 4 shows detail of the preferred embodiment;

FIG. 5 shows the preferred embodiment in the (x, y) plane;

FIG. 6 illustrates the principle involved in the invention;

FIG. 7 shows the results of a SIMION simulation;

FIG. 8 shows a comparison with the prior art;

FIG. 9 shows a simplified model of ions motion in the device disclosed;

FIG. 10 shows the results of an analytical model describing the ionseparation within the device;

FIG. 11 shows a graph of resolution versus mobility value;

FIG. 12 shows the results of an analytical model describing the ionseparation within the device;

FIG. 13 shows a graph of resolution versus mobility value;

FIG. 14A shows a plan view and FIG. 14B shows a side view of anotherembodiment of the invention;

FIG. 15A shows a plan view and FIG. 15B shows an end view of anotherembodiment of the invention;

FIG. 16 shows a plan view of another embodiment of the invention; and

FIG. 17 shows a plan view of another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be describedwith reference to FIG. 1. It should, however, be understood that thepresent invention is not restricted to an ion mobility spectrometer andnumerous alternative geometries to the annular geometry shown in FIG. 1are intended to fall within the scope of the present invention.

According to an embodiment of the present invention an ion mobilityseparator may be provided comprising two concentric cylinders as shownin FIG. 1. The cylinders comprise an outer cylinder 1 and an innercylinder 2 defining an annular volume through which ions pass in use.The concentric cylinders are preferably made of a non conductivematerial. The ion mobility separator preferably has an entrance end 3through which ions enter the ion mobility separator and an exit end 4through which ions exit the ion mobility separator.

The annular volume between the two concentric cylinders 1,2 ispreferably supplied with a buffer gas or drift gas such as helium ornitrogen. For example, the pressure of drift gas in the annular volumemay be set to between 0.5 torr to 5 torr.

According to a specific embodiment a spiral of conductive material maybe attached to the outside of the inner cylinder 2 as shown in FIG. 2A.A spiral of conductive material may also be attached to the innersurface of the outer cylinder 1 as shown in FIG. 2B. The inner and outerspirals are preferably arranged such that the inner spiral follows thepath of the outer spiral over the length of the device. As an example,if the angle θ of the conductive strip with respect to the y axis asshown in FIG. 2A is 20° and the diameter of the inner cylinder 2 is 20mm, then the strip will make one complete revolution of the cylinderevery 22.87 mm. For a 100 mm cylinder length the wire would make 4.37complete revolutions of the cylinder. The overall length of theconductive track would be approximately 292 mm. If the diameter of theouter cylinder 1 were 30 mm then an annulus of 5 mm radial width will beformed between the outer cylinder 1 and the inner cylinder 2.

FIG. 3 shows the inner cylinder 2 and the outer cylinder 1 each in thez,y direction with complimentary spiral conductive strips. A first DCpower supply 5 is provided to allow the potential of the conductivestrip to be raised thereby producing a helical DC potential barrieralong the annular volume. The resulting DC potential barrier isindicated by the shaded area between the inner and outer spiral tracksin FIG. 3.

The conductive strip may be resistive and such that the total resistanceof the strip may be of in the order of 50-500Ω. This allows a potentialgradient to be applied from one end of the conductive strip to the otherusing a second DC supply 6.

FIG. 4 shows an embodiment wherein the outer cylinder 1 comprises sixequally spaced conductive resistive strips a,b,c,d,e,f forming sixspirals. Using the example of a 20 mm diameter inner cylinder and a 20degree angle θ, for a strip width of 0.5 mm and gap between strips of 1mm a total of 15 strips may be applied to the inner cylinder 2.

In operation the second DC supply 6 may be arranged to apply a DCpotential to all six conductive strips effectively producing a DC fieldwhich acts to urge ions from the entrance end 3 of the ion mobilityseparator to exit end 4 of the ion mobility separator. However, thefirst DC supply 5 is preferably only applied to one of the spiralconductive strips at any specific time.

The same potentials are preferably applied to the complimentaryconductive strips on the inner cylinder 2.

In the preferred embodiment ions may be confined radially in the annularvolume 3 by application of an AC voltage oscillating at RF frequency tothe conductive strips. The RF voltage between adjacent strips ispreferably 180° out of phase.

FIG. 5 shows a view of the outer cylinder 1 and inner cylinder 2 in thex,y direction illustrating how the confining RF and the spiral DCbarrier may be applied to the inner cylinder 2 and outer cylinder 1.

Both the inner cylinder 2 and outer cylinder 1 may be made from madefrom a dielectric material with conductive strips on the inside andoutside of each cylinder. For example a flexible PCB construction may beused.

DC electrodes a,b,c,d,e,f are shown in FIG. 5 on the outside of theinner cylinder 2 and on the inside of the outer cylinder 1.

Also shown in FIG. 5 are auxiliary conductive strips running parallel toelectrodes a,b,c,d,e,f but on the opposite side of the cylinders ofdielectric material. The auxiliary strips preferably follow the samespirals as described for each of the electrodes a,b,c,d,e,f down thelength of the device. The auxiliary electrodes may be supplied with ACpotential oscillating at RF frequency with adjacent electrodespreferably supplied with opposite phases of RF. The AC potentialpreferably capacitively couples onto to the DC electrodes via thedielectric material resulting in a radial RF confining field within theannular volume. The AC voltage may, for example, have an amplitude of400 V pk to pk and a frequency of 1 MHz.

In operation ions are preferably pulsed into the ion mobility separatorat a time T1 via the entrance 3 of the ion mobility separator. At timeT1 the potential applied to strip f as shown in FIGS. 4 and 5 ispreferably raised above the potential of the other five spiral stripsthereby effectively forming a barrier through which ions may not pass.Ions are preferably arranged to enter the ion mobility separator in aregion away from conductive strip f so that their initial transit intothe ion mobility separator is not impeded.

The potential gradient applied across each strip by the first DC voltagesupply 5 preferably urges ions to progress down the ion mobilityseparator in a spiral path following the path DC barrier produced by thepotential applied to strip f.

The component of the field which results in separation of the ions ispreferably orthogonal to the DC barrier.

Ions passing through the buffer gas are preferably driven in a spiralpath by the spiral DC gradient and will separate according to their ionmobility.

As the ions travel down the device at a subsequent time T2, where T2>T1,the DC voltage applied to strip f is preferably removed and a DC voltageis preferably applied only to strip e as shown in FIG. 4. Therefore, attime T2 DC the potential of strip e is preferably raised with respect tothe other conductive strips. At a subsequent time T3, where T3>T2, theDC potential barrier is preferably switched to conductive strip d only.At a subsequent time T4 the DC potential is applied only to strip c. Ata subsequent time T5 the DC is preferably applied only to strip b. At asubsequent time T6 the DC is preferably applied only to strip a.Finally, at a subsequent time T7, wherein T7>T6>T5>T4>T3>T2>T1, the DCis preferably applied only to strip f. This pattern is preferablyrepeated throughout the analysis.

Applying the DC voltage to strips a,b,c,d,e,f in this way creates atravelling DC helix or a rotating DC helix acting to oppose the motionof ions through the device as they are urged through the drift gas bythe DC potential gradient applied to each conductive strip. Ions arepreferably urged back towards the entrance of the device by the DChelical travelling wave but, at the same time, the ions slip down theinclined helical wave front as they are moved towards the exit of thedevice by the applied DC gradient.

By adjusting the speed of the travelling helical wave or the strength ofthe DC field the residence time of ions within the device can beextended. Ions experience a DC field acting towards the exit of thedevice for a longer period of time than would be expected if no helicaltravelling wave were present. This longer residence time is equivalentto the ions travelling through a proportionately longer ion mobilityseparation device and hence the ion mobility separation power isincreased compared to a conventional ion mobility separation device of acomparable physical length.

To aid conceptualisation of the principle of operation of the devicedescribed, a simple model may be imagined wherein the travelling helicalDC barrier is stretched out to form a linear barrier existing on a flatplane. FIG. 6 represents a single ionic species within such a model. Thediagonal line labelled T1 represents a DC barrier existing at time T1over which ions cannot pass. In this diagram the barrier traverses aflat plane with the height of the barrier extending vertically towardsthe viewer. This barrier is inclined with respect to the plane of theion detector or exit of the device D by the angle θ. The length Ldefines the length of the device from the entrance plane S to the exitplane D. An electric field E exists over the length L of the deviceurging ions from the entrance to the exit plane. In this model ionsexist in an elevated pressure of buffer gas such that ion motion isdominated by the ion mobility K.

At time T1 ions are preferably urged in a direction along the inclinededge of the DC barrier by the component of the DC force E resolved inthis direction. If the DC barrier at T1 is static then ions willeventually reach the exit of the device D after travelling a distance Lin field E. The time for the ions to travel from the entrance to theexit (DTs) will depend on the mobility of the ion and will be given by:

$\begin{matrix}{{DTs} = \frac{L}{KE}} & (2)\end{matrix}$

However, in FIG. 6 the ion is allowed to travel distance d1 through thedevice until time T2. In this example d1=L/4.

At time T2 the DC barrier is preferably moved upwards towards theentrance of the device and now exists at the position labelled T2. Theion preferably moves on the front of this barrier in a direction normalto the barrier. Ions of all mobility values are preferably moved bysubstantially the same amount. As shown, the ion has been moved towardsthe entrance of the device and again starts to move down the barrierwhich is now shown in position T2. The ion is preferably allowed totravel a distance d2, where d2=d1, under the influence of electric fieldE until time T3. At time T3 the barrier has again been moved and the ionhas been pushed back towards the entrance. The ion is again allowed todrift a distance d3, where d1=d2=d3, until time T4 where the barrier isagain moved. The ion is then allowed to travel distance d4, whered1=d2=d3=d4, before the barrier is moved again at time T5. The ion isthen allowed to travel distance d5, where d1=d2=d3=d4=d5, before thebarrier is moved again at time T6. Finally, the ion is allowed to move adistance d6, where d1=d2=d3=d4=d5=d6 at which point the ion exits thedevice.

The overall distance that the ion has travelled in the field E (Lt) is:Lt=6×d1  (3)

This can be compared to the distance over which the ion would havetravelled in the same field if the inclined barrier had not been movedL:L=4×d1  (4)

Thus the absolute length of the mobility separation device L can beextended to a longer virtual length Lt. The amount that the drift lengthis extended is a function of the ions mobility.

To investigate the performance of this technique in terms of mobilityseparating power, a model similar to the theoretical model described inFIG. 6 was constructed using SIMION® ion optic simulation software.

The parameters used in this model were length of drift region L is 100mm, height of DC barrier is 100V, angle of DC barrier wrt exit planeθ=20°, pressure of nitrogen (hard sphere model) P is 0.5 Torr, speed ofbarrier normal to wave front is 40 m/s and driving field E is 4 V/mm.

The trajectories of four ions were modelled. All ions were singlycharged with mass to charge ratio 500 but with differing mobilityvalues. Ion #1 had K=0.173 M²V⁻¹s⁻¹ and cross section of 200 Å²; Ion #2had K=0.139 M²V⁻¹s⁻¹ and cross section of 250 Å²; Ion #3 had K 0.126M²V⁻¹s⁻¹ and cross section of 275 Å²; and on #4 had K=0.115 M²V⁻¹s⁻¹ andcross section of 300 Å².

Groups of 1000 ions of each mobility value were modelled and the meandrift time and the standard deviation in drift time for each species wasrecorded.

FIG. 7 shows the results from SIMION® for this model. Peaks arerepresented as Gaussian peaks with mean and standard deviations measuredfrom the output of the simulation.

FIG. 8 shows the theoretical IMS spectrum of the same group of ionsseparated in a standard IMS drift tube at the same pressure, length andfield as the example modelled in FIG. 7.

There is clearly a marked improvement in IMS separation power using thetechnique according to the preferred embodiment. This is accompanied asexpected by an increase in the drift time for all the species.

It can also be seen, by comparison of FIGS. 7 and 8, that the differencein drift time for the ions of high mobility is less than the differencefor the low mobility ions. This demonstrates that the increase in thedrift time for these ions is dependent on the mobility.

Because the drift time is not linearly dependent on the mobility of theions, the mobility resolution R can no longer be calculated directlyfrom the expression:

$\begin{matrix}{R = {\frac{DT}{\Delta\;{Dt}} = \frac{K}{\Delta\; K}}} & (5)\end{matrix}$

To produce an expression for the resolution of the device an analyticalexpression is required describing the motion of the ions within thetravelling helical wave.

FIG. 9 shows a further simplification illustrating the principal of thepreferred embodiment. A drift tube 8 of length L is supplied with afield E urging ions along the tube 8 through a buffer gas with velocityV wherein:V=K·E  (6)

A detector 9 is placed initially at the end of the drift tube 8 andmoves away from the exit of the drift tube 8 with a constant velocity Vdeffectively increasing the total length of the tube L_(tot) as the ionsdrift. The field E is assumed to be constant throughout the ions journeyto the detector 9.

The time t_(tot) at which an ion reaches the receding detector 9 isgiven by:

$\begin{matrix}{t_{tot} = \frac{L}{{KE} - V_{d}}} & (7)\end{matrix}$

The extent to which a packet of ions introduced into the drift cellspreads with time is governed by diffusion within the buffer gas.

The standard deviation in the width if the ion packet with time σL isgiven by:

$\begin{matrix}{{\sigma\; L} = \sqrt{\frac{2\;{kTKt}}{q}}} & (8)\end{matrix}$wherein k is Boltzmann constant, T is the temperature in Kelvin, t isthe drift time and q is the charge on the ion.

The standard deviation in terms of time at for the system may found bydivision of Eqn. 8 by the relative velocity of the ion given by Eqn. 7:

$\begin{matrix}{{\sigma\; t} = {\sqrt{\frac{2\;{kTKt}}{q}} \cdot \frac{t}{L}}} & (9)\end{matrix}$

The FWHM of the mobility peak in time Δt is:Δt=2√{square root over (2 ln 2)}·σt  (10)

To compare the results from the SIMION® model described an additionalfactor must be applied to represent the ion arriving at the detector 9at a non normal angle θ. This has the effect of reducing the observedFWHM peak width by a factor F, wherein F=sin(θ).

FIG. 10 shows the results from this approach for the same ions Ion #1,Ion #2, ion #3 and ion #4 as shown in FIGS. 7 and 8.

In this model the drift region L was modelled as 100 mm, the velocityV_(d) of the detector was 29 m/s and the driving field E was 4 V/mm.

There is very good agreement with the results from the SIMION®simulation shown in FIG. 7 suggesting that this is a very goodanalytical approximation.

To derive an expression for mobility resolution using this model, Eqn. 2may be rewritten as:

$\begin{matrix}{R = {\frac{K}{\Delta\; K} = {{\frac{K}{\Delta\; t} \cdot \frac{{dt}_{tot}}{dK}}}}} & (12)\end{matrix}$

Substituting Eqns. 7 and 8 into Eqn. 9 and evaluating yields anexpression for the mobility resolution R of the device for a givendetector velocity. The detector velocity is analogous to the speed ofrotation of the helical DC barrier in the preferred embodiment:

$\begin{matrix}{R = \frac{E\sqrt{K \cdot L \cdot q}}{\sqrt{16\;{\ln(2)}{{kT}\left( {{KE} - V_{d}} \right)}} \cdot {\sin(\theta)}}} & (13)\end{matrix}$

FIG. 11 shows a plot of mobility resolution R versus K value. The solidline 10 represents the theoretical resolution from the device under theconditions used in the SIMION® simulation and analytical modeldescribed.

The dashed line 11 shows the resolution obtained from a standard DCdrift tube of the same length L operating at the same pressure and fieldE. A mobility resolution of 13 was calculated for the standard drifttube. Increases in resolution of between 3 and 5 are achieved over thisrange of mobility according to the preferred embodiment. To achievethese resolutions at the same pressure and field using a standard drifttube a drift tube length of between 0.9-2.5 m would be required which isimpractical.

The mobility resolution is dependent of the mobility of the ion as thetotal effective distance the ions travel in the applied field depends onthe ions mobility.

In the embodiment described, ions are driven towards the exit of thedevice by the DC field acting along the helical DC barrier and are movedback towards the exit of the device by sweeping or rotating the DCbarrier. Ions eventually exit in the direction in which they are urgedby the DC field and may be detected. However, improved IMS separationmay also be achieved by driving ions such that they exit in the oppositedirection i.e. in the direction in which they are urged by thetravelling or rotating helical DC barrier.

FIG. 12 shows the results of a SIMION® ion optical simulation for themodel as described for FIG. 7. However, the speed of the travellingbarrier was increased from 40 m/s to 94 m/s. Ion arrival times arerecorded as the ions exit the device in the direction of the wave front.

The order in which the four species modelled elute from the device isreversed with ions having relatively low ion mobility exiting the devicebefore ions with higher mobility.

FIG. 13 shows a plot of mobility resolution R versus mobility Kcalculated using a very similar analytical approach as described forFIG. 10. However, in this model the detector is arranged at the entranceto the device and ‘catches up’ with the ions as they separate in the DCfield. The resolution for a standard drift tube under the sameconditions is shown as a dotted line 11. The resolution for thisstandard drift tube is 13.

It should be noted that that during IMS separation the amplitude of theDC field may be ramped or stepped over a range of values and/or thevelocity of the travelling spiral DC barrier may be ramped or steppedover a range of values. In this mode of operation the resolving power ofthe device may be maximised for specific mobility ranges.

In another mode of operation an additional DC or RF barrier may bearranged at one end of the device (or at some point along the length ofthe device) such that once ions have entered the device they cannot exitor travel past this point. By adjusting the magnitude of the DC fieldand the speed of the opposing travelling DC helix, all ions may bedriven to the end of the device and can effectively be trapped at aspecific location within the device. Ions may be trapped by acombination of the exit barrier and the DC travelling spiral or by theexit barrier and the DC field acting along the length of the DC spiral.

Once ions are trapped they can be scanned out by ramping or stepping theamplitude of the DC field or by ramping or stepping the velocity of thetravelling helical DC barrier or by a combination of both. In this modeof operation the resolving power of the device may be maximised for awide range of mobilities.

By varying the driving forces within the device it is possible toarrange ions with different mobility ranges to exit via different endsof the device simultaneously if desired.

In the preferred embodiment ions enter the device at one end of theannular volume, however, ions may be arranged to enter the annularvolume at any point along the length of the device via a separate ionentrance through the outer cylinder.

The device may also be operated as an ion guide without any IMSseparation according to another mode of operation. In this mode ofoperation the DC field acting to urge ions along the DC spiral barriermay be effectively set to zero and the travelling DC spiral may be usedto drive ions through the device. The travelling or rotating DC spiralmay be used to transmit a continuous, discontinuous or pulsed ion beam.

The device will operate as an RF ion guide over a wide range of buffergas pressure. The transit time of ions through the device may beprecisely controlled by adjusting the speed of the travelling DC spiralas described.

At an elevated buffer gas pressure the travelling spiral DC barrier cantransport a continuous beam of ions or a discontinuous beam of ions withno effective mobility separation. This is not possible using a staticaxial DC driving force.

At an elevated buffer gas pressure the travelling spiral DC barrier cantransport a continuous beam of ions or a discontinuous beam of ions withno effective mobility separation and no partitioning of the ion beam.This is not generally possible using a conventional DC travelling wavegas cell.

When used with a continuous ion beam the device may be used as a high orlow mobility cut off filter. For example, the speed of the travelling DChelical barrier opposing the DC field may be adjusted so that only ionshaving an ion mobility above a certain mobility value will exit thedevice. Ions of lower mobility will not be driven towards the exit ofthe device or will be driven back towards the exit.

Although the preferred embodiment has a cylindrical geometry otherembodiments with planar and other geometries are also intended to fallwithin the scope of the present invention.

FIG. 14A shows a plan view of another embodiment with a planar geometryshown in the x,y plane. This embodiment is similar to FIG. 6. Horizontalplanar electrodes are segmented into a plurality of electrode segments.Ions enter the device at entrance 3 and are driven along a horizontaldiagonal DC barrier 14 by an electric field E. The position of thediagonal DC barrier is swept with time in the direction shown by thearrow 15. Ions of different mobility will exit at different points alongthe length of the device W.

FIG. 14B shows a side view of the device in the x,z plane. Ions arepreferably confined vertically within the array of electrodes by apseudo-potential well due to an RF voltage being applied to the upperand lower arrays of segmented electrodes.

Although this is not as compact a design as the preferred embodiment,the mobility resolution which can be achieved for a given length L issignificantly higher than can be achieved with a standard drift tube oflength L with a field E.

Another planar embodiment is shown in FIG. 15A in the x,y direction.FIG. 15A shows a plan view. Ions enter at entrance 3 and are drivenalong a diagonal DC barrier 14 by an electric field E. The DC barrier isdiscontinuous over the length of the device L allowing ions to reversetheir direction in the x axis as they travel through the device. Thepositions of the multiple DC barriers are swept with time in thedirection shown by the arrow 15.

FIG. 15B shows an end view of the device in the x,z direction. Ions arecontained in the z (vertical) direction by supplying the upper and lowerarrays of segmented electrodes supplied with RF voltage which results ina pseudo-potential well. Ions are contained in the x direction byelectrodes 15 supplied with DC voltage.

Various different methods of containing ions in the z and x directionsfor the embodiments shown in FIGS. 14 and 15 are contemplated.

FIG. 16 shows a plan view of a representation of another embodiment witha planar geometry in the x,y. In this case ions enter at entrance 3 andare driven along a planar spiral DC barrier by an electric field E whichacts radially towards the centre of the spiral. The position of thespiral DC barrier is moved with time such that the spiral rotates aroundits central point in the direction indicated by the arrow 16. The spiralshown in FIG. 16 represents the location of a DC potential barrier.

Ions eventually exit the device at the central point 17. In thisembodiment ions may be injected at the centre of the spiral and driventowards the outside of the spiral by increasing the speed of rotation ofthe spiral DC barrier or by decreasing the field E sufficiently.

It should be noted that the direction of both the radial field and thedirection of rotation of the spiral may be reversed without changing theprinciple of operation.

As for the embodiments in FIGS. 14 and 15 ions may be contained in thex,z (vertical) plane using an array of RF electrodes.

In the embodiments described in FIGS. 14, 15 and 16 the total time thations spend in the electric field and hence the resolution of the devicemay be adjusted, in a mobility dependant way, by changing the speed ofmovement of the DC barrier or by ramping the electrostatic field.

FIG. 17 shows another embodiment similar to that shown in FIG. 16. Inthis embodiment the device acts within ion trapping properties. Ionsenter the device at entrance 3 and are driven in the direction shown byarrow 18 along a partial spiral barrier by a combination of theelectrostatic field E acting radially towards the centre of the deviceand the force applied to the ions by rotation of a DC potential spiralbarrier in the direction shown by arrow 16. In this case the speed ofrotation of the DC potential spiral barrier is chosen such that ions aredriven towards the outside of the spiral. When ions reach the end of thespiral 19 they are driven by the electric field E between one end of thespiral 19 and a point near the other end of the spiral 20. They thencontinue to cover the same path round the spiral back to point 19. Theions are effectively trapped on the edge of the spiral barrier andseparate as they proceed to revolve around the central point 17. As theions do not take a unique path ions with low mobility will eventuallycatch up with the ions of higher mobility which travel around the devicewith a lower relative velocity. Ions may be ejected from the device, forexample, by removing a portion of the barrier at point 20 and byallowing ions to fall to the centre or by removing the field E actingradially towards the centre of the device. In the latter case ions willexit in a direction indicated by the arrow 21. According to thisembodiment ions may be arranged to exit the device in reverse order ofion mobility.

Various modifications to the embodiments shown and described above arecontemplated.

For example, modifications may be made to the way in which the time andposition varying DC barrier is applied. For example, rather thanapplying the potential in a series of discreet steps with potentialssuddenly appearing and disappearing on different elements of the device,the amplitude of the DC potential may be applied and then removed usinga smooth continuous function. This allows for a smoother transitionbetween the travelling waves experienced by the ions.

In the preferred embodiment described above the potential forming thetravelling DC barrier is shown and described as being applied to onlyone conductive strip at a time. However, it may also be advantageous toapply the potential to groups of adjacent conductive strips. Otherembodiments are contemplated which affect the magnitude and the shape ofthe travelling wave.

The confining RF voltage may be applied in other ways than described inthe preferred embodiment. For example, the RF confining potential may beapplied between the complimentary spirals on the inner and outercylinder such that the conductive strips on the inner cylinder aresupplied with RF potential 180 degrees out of phase with the strips onthe outer cylinder. All strips on the inner cylinder may be arranged tobe at the same phase and all strips on the outer cylinder may bearranged to be at the same phase.

Alternatively, the RF confining potential may be applied such thatspirals on the inner cylinder have opposite phases to the complimentaryspirals on the outer cylinder. However, adjacent spirals on the innerand the outer may have opposite phases of RF.

Other schemes may be envisaged resulting in RF confinement.

The geometry of the device need not have a circular cross section.Elliptical, rectangular or Irregular cross sections can be envisaged.

The inner component shown in FIG. 1 need not be concentric with theouter component.

The pitch of the travelling helical barrier described by the angle θ inFIG. 2 may vary along the length of the device. This effectively changesthe speed at which ions are driven through the device by the travellinghelical barrier with position along the device.

The driving force opposing the travelling DC helix may be a massdependent force such as a pseudo potential force rather than a DC field.A pseudo potential driving force may be created by applyingprogressively higher amplitude of RF voltage with distance along thelength of the device or by reducing the gap between the inner and theouter cylinder from the entrance end to the exit end.

In this case separation will be related to both the mobility and themass to charge ratio of the ions.

It is possible to construct the device to produce the required fields indifferent ways than described in relation to the preferred embodiment.For example, the inner and outer cylinders may be concentric RF ringstacks with opposite phases of RF on adjacent rings. If these rings arethemselves radially segmented then appropriate static and time varyingDC potentials may be applied to the segments to produce the requiredfields.

Using this type of construction it is possible to apply a moretraditional DC travelling wave to the device. The combination of atravelling DC helix opposed by a DC travelling wave may be used forenhanced mobility separation or mass separation as described inWO2008/071967.

Additionally using a multiply segmented ring stack a travelling pseudopotential helical barrier may be produced by altering the amplitude ofRF applied to each segment in a time varying manner.

More than one travelling helical DC barrier may be simultaneouslyimposed over the length of the device creating more than one uniquepathway for ions to travel down the device.

In another embodiment, this device may be used as a cylindricaldifferential ion mobility device (DIMS) device. In this embodiment theRF confining potential may be asymmetric or an additional asymmetricwaveform may be applied to the RF confining electrodes between the innerand outer cylinder. In this embodiment the RF confining potential isapplied between the complimentary spirals on the inner and outercylinder such that the conductive strips on the inner cylinder aresupplied with RF potential 180 degrees out of phase with the strips onthe outer cylinder. All strips on the inner cylinder are the same phaseand all strips on the outer cylinder are the same phase.

This results in dispersion of the ions in the radial direction betweenthe inner and outer cylinders with respect to their differential ionmobility.

Only ions within a specific band of differential ion mobilities will becontained within the device. Other ions will be lost to the electrodesof the inner or outer cylinder. By applying an additional DC voltagebetween the outer and the inner cylinders (compensation voltage) ionswith different differential mobility can be arranged to be transferredthough the device.

If, during differential ion mobility separation, ions are driven throughthe device by the helical travelling wave with no DC field, ions willonly be separated in the radial direction. Ions may be introduced intothis device as a continuous beam or as a pulse or packet of ions.

If DC field or a combination of DC and opposing helical travelling waveis used to drive ions through the device ions will separate radiallywith respect to their differential mobility and along the length of thespiral with respect to ion mobility.

The device can also be used in conjunction with another separationdevice (for example ion mobility or mass separator) or filter upstreamor downstream of the device.

If a second IMS separation device is placed upstream of the devicedisclosed, the output of the first device can be synchronised to thechanges in the speed of the helical DC travelling wave or the DC fieldstrength to allow the IMS resolution to be optimised over a wide rangeof mobilities.

The embodiments disclosed may used in conjunction with each other or asmultiple devices is series with ions passing from one device to another.For example, multiple devices constructs a shown for the planar spiralgeometry in FIG. 6 may be stacked one on top of each other. Ions in theupper most spiral may be driven under a chosen set of conditions towardsthe centre of the spiral 17. The ions may then be directed, by a DCfield for example, to the entrance of another spiral device directlybelow the first and separation of ions can continue down this secondspiral. If the entrance to the second device is also at the centre ofthe spiral of the second device the direction of the field and thedirection or rotation of the spiral barrier are reversed. In the exampleshown in FIG. 16 the field E will act radially in the direction from thecentre of the spiral to the outside. By stacking multiple devices inthis way the resolution and range of mobility values over which canreside in the device can be increased.

The devices disclosed may be used to analyse both positive and negativeions simultaneously. If positive ions are introduced at one end of thedevice and negative ions are introduced at the opposite end of thedevice these ions will travel in opposite directions and exit the deviceat opposite ends. Positive ions will take will take a unique paththrough the device compared to negative ions because of the nature ofthe travelling DC barriers. It is possible to use this characteristic topopulate the device with both positive and negative ions and controltheir interaction time by removing or reducing the DC barrier. In thisway ion-ion interactions such as electron transfer dissociation ETD.Hydrogen deuterium exchange HDX, or charge reduction may be performed.

It is possible to trap both positive and negative ions using theembodiments of the travelling DC barrier described by arranging ions tobe urged towards the centre of the device by barriers rotating or movingin opposite directions at either end of the device.

Although the present invention has been described with reference to thepreferred embodiments, it will be understood by those skilled in the artthat various changes in form and detail may be made without departingfrom the scope of the invention as set forth in the accompanying claims.

The invention claimed is:
 1. Apparatus for separating ions according toone or more physico-chemical properties, wherein said apparatus isarranged and adapted to create an ion channel in which ions are confinedin use, wherein ions are confined within said ion channel in a firstplane or confinement direction by DC voltages, DC potentials orelectrostatic potentials and ions are confined within said ion channelin a second plane or confinement direction by RF voltages, RF potentialsor pseudo-potentials, wherein said second plane or confinement directionis substantially orthogonal to said first plane or confinementdirection; wherein said ions are caused to separate according to saidone or more physico-chemical properties along an axis of said ionchannel or along said ion channel towards a first end and wherein saidapparatus is further arranged and adapted to move said axis of said ionchannel or said ion channel away from said first end, wherein an effectof moving said axis of said ion channel or said ion channel away fromsaid first end is to increase an effective path length along which ionsseparate according to said one or more physico-chemical properties; andwherein said apparatus comprises: a first device arranged and adapted tocause ions to separate according to said one or more physico-chemicalproperties in a first direction or along said axis of said ion channelor along said ion channel with a velocity which is substantiallydependent upon said one or more physico-chemical properties; and asecond device arranged and adapted to drive said ions in a seconddirection with a velocity which is substantially independent of said oneor more physico-chemical properties.
 2. Apparatus as claimed in claim 1,wherein said axis of said ion channel or said ion channel is non-linear.3. Apparatus as claimed in claim 1, wherein said axis of said ionchannel or said ion channel is helical, spiral or curved.
 4. Apparatusas claimed in claim 1, wherein said axis of said ion channel or said ionchannel is linear.
 5. Apparatus as claimed in claim 1, wherein saidfirst end comprises an exit and is opposed to a second end through whichions initially entered said apparatus.
 6. Apparatus as claimed in claim1, wherein said first end comprises an entrance through which ionsinitially entered said apparatus.
 7. Apparatus as claimed in claim 1,wherein said ion channel comprises a DC potential well.
 8. Apparatus asclaimed in claim 1, wherein said ion channel is formed between: (i) afirst DC voltage gradient, a first DC potential, a first electrostaticbarrier, a first DC potential barrier or a first pseudo-potential; and(ii) a second moving DC potential barrier, a second moving electrostaticbarrier, a second moving DC potential barrier or a second movingpseudo-potential barrier.
 9. Apparatus as claimed in claim 1, whereinsaid first device is arranged and adapted: (i) to maintain a first DCvoltage gradient, a first DC potential or a first DC electric fieldacross at least a portion or at least 10%, 20%, 30%, 40%, 50%, 60%, 70%,80% or 90% of the axial length of said apparatus or along said axis ofsaid ion channel or along said ion channel; or (ii) to apply a firstelectrostatic potential or force along at least a portion or at least10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of the axial length ofsaid apparatus or along said axis of said ion channel or along said ionchannel; or (iii) to apply one or more first transient DC voltages orpotentials to a plurality of electrodes in order to urge ions along atleast a portion or at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or90% of an axial length of said apparatus or along said axis of said ionchannel or along said ion channel; or (iv) to apply a first RF voltagecomprising three or more phases to a plurality of electrodes, whereindifferent electrodes are connected to different phases of said RFvoltage and wherein said first RF voltage urges ions along at least aportion or at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of theaxial length of said apparatus or along said axis of said ion channel oralong said ion channel; or (v) to apply a first pseudo-potential orforce wherein an amplitude or frequency of an RF voltage applied to aplurality of electrodes varies, increases or decreases along at least aportion or at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of theaxial length of said apparatus or along said axis of said ion channel oralong said ion channel.
 10. Apparatus as claimed in claim 9, whereinsaid first device is arranged and adapted to apply or maintain a firstelectrostatic potential or force, a first DC potential or force, or afirst pseudo-potential or force along at least a portion of saidapparatus or along said axis of said ion channel or along said ionchannel in order to cause ions to separate according to said one or morephysico-chemical properties.
 11. Apparatus as claimed in claim 10,wherein said second device is arranged and adapted to apply or maintaina second moving electrostatic potential barrier, a second moving DCpotential barrier or a second moving pseudo-potential barrier along atleast a portion of said apparatus in order to drive said ions in saidsecond direction.
 12. Apparatus as claimed in claim 11, wherein saidfirst electrostatic potential or force, said first DC potential orforce, or said first pseudo-potential or force is substantially greaterthan, equal to, or less than said second moving electrostatic potentialbarrier, said second moving DC potential barrier or said second movingpseudo-potential barrier.
 13. Apparatus as claimed in claim 11, wherein:(i) said first electrostatic potential or force is substantially greaterthan, equal to, or less than said second moving electrostatic potentialbarrier; or (ii) said first DC potential or said first DC electric fieldis substantially greater than, equal to, or less than said second movingDC potential barrier; (iii) said first transient DC voltages orpotentials have an amplitude which is substantially greater than, equalto or less than said second transient DC voltages or potentials; or (iv)said first RF voltage has an amplitude which is substantially greaterthan, equal to or less than said second RF voltage; or (v) said firstpseudo-potential or force is substantially greater than, equal to, orless than said second pseudo-potential barrier.
 14. Apparatus as claimedin claim 1, wherein said second device is arranged and adapted: (i) toapply a second moving electrostatic potential barrier along at least aportion or at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of theaxial length of said apparatus; or (ii) to apply one or more secondtransient DC voltages or potentials to a plurality of electrodes inorder to urge ions along at least a portion or at least 10%, 20%, 30%,40%, 50%, 60%, 70%, 80% or 90% of the axial length of said apparatus; or(iii) to apply a second RF voltage comprising three or more phases to aplurality of electrodes, wherein different electrodes are connected todifferent phases of said RF voltage and wherein said second RF voltageurges ions along at least a portion or at least 10%, 20%, 30%, 40%, 50%,60%, 70%, 80% or 90% of the axial length of said apparatus; or (iv) toapply a second moving pseudo-potential barrier wherein the amplitude orfrequency of an RF voltage applied to a plurality of electrodes varies,increases or decreases along at least a portion or at least 10%, 20%,30%, 40%, 50%, 60%, 70%, 80% or 90% of the axial length of saidapparatus.
 15. Apparatus as claimed in claim 1, wherein said firstdevice is arranged and adapted to drive or urge ions in said firstdirection with a first velocity or a first component of velocity andsaid second device is arranged and adapted to drive or urge ions in saidsecond direction with a second velocity or second component of velocity,wherein said first velocity or first component of velocity issubstantially greater than, equal to or less than said second velocityor said second component of velocity.
 16. Apparatus as claimed in claim11, wherein said first electrostatic potential or force, said first DCpotential or force, or said first pseudo-potential or force has anon-zero component: (i) in a direction which is substantially parallelto said second moving electrostatic potential barrier, said secondmoving DC potential barrier, or said second moving pseudo-potentialbarrier at a particular instance in time; or (ii) in a directionsubstantially orthogonal to a direction of movement of said secondmoving electrostatic potential barrier, said second moving DC potentialbarrier, or said second moving pseudo-potential barrier.
 17. Apparatusas claimed in claim 1, wherein said first device drives ions with afirst velocity or force in said first direction and wherein said seconddevice drives ions with a second velocity or force in said seconddirection, wherein either: (i) said first direction is inclined to oroffset relative to said second direction; or (ii) said first directionis not co-directional with said second direction; or (iii) said firstdirection is not counter to said second direction; or (iv) said firstvelocity or force has a non-zero component of velocity or force in adirection substantially orthogonal to said second direction. 18.Apparatus as claimed in claim 1, wherein said first device drives ionswith a first velocity or force and said second device drives ions with asecond velocity or force, wherein either said first velocity or force orsaid second velocity or force varies with time or position. 19.Apparatus as claimed in claim 1, wherein said physico-chemical propertycomprises ion mobility.
 20. Apparatus as claimed in claim 19, whereinsaid apparatus comprises an ion mobility spectrometer or separator. 21.Apparatus as claimed in claim 1, wherein said physico-chemical propertycomprises differential ion mobility.
 22. Apparatus as claimed in claim21, wherein said apparatus comprises a differential ion mobilityspectrometer or a Field Asymmetric Ion Mobility Spectrometer (“FAIMS”)device.
 23. Apparatus as claimed in claim 1, wherein saidphysico-chemical property comprises mass or mass to charge ratio. 24.Apparatus as claimed in claim 23, wherein said apparatus comprises amass or mass to charge ratio analyser.
 25. Apparatus as claimed in claim1, wherein said apparatus comprises: an inner cylinder and an outercylinder, wherein said inner cylinder and said outer cylinder define anannular volume through which ions are transmitted in use; and whereinone or more spiral or helical electrodes are arranged on a surface ofsaid inner cylinder or on a surface of said outer cylinder. 26.Apparatus as claimed in claim 25, wherein: said first device is arrangedand adapted to maintain a DC electric field or a pseudo-potential forcewhich acts to urge ions from a first end of said apparatus to a secondend of said apparatus; and said second device is arranged and adapted toapply one or more transient DC voltages to said one or more spiral orhelical electrodes in order to urge ions towards said first end of saidapparatus.
 27. Apparatus as claimed in claim 25, wherein said innercylinder or said outer cylinder have a circular, elliptical,non-circular, rectangular or irregular cross section.
 28. Apparatus asclaimed in claim 25, wherein a pitch of said one or more spiral orhelical electrodes is constant, increases, decreases or varies along thelength of said apparatus.
 29. Apparatus as claimed in claim 25, furthercomprising a device for applying an RF voltage to said one or morespiral or helical electrodes in order to confine ions radially withinsaid annular volume.
 30. Apparatus as claimed in claim 25, furthercomprising a device arranged and adapted to supply a buffer gas to saidannular volume between said outer cylinder and said inner cylinder. 31.Apparatus as claimed in claim 25, wherein said inner cylinder or saidouter cylinder comprise a non-conductive or dielectric material. 32.Apparatus as claimed in claim 25, further comprising one or moreauxiliary electrodes provided on an opposite side of said inner cylinderor said outer cylinder to that of said one or more spiral or helicalelectrodes.
 33. Apparatus as claimed in claim 32, wherein an RF voltageis applied to said one or more auxiliary electrodes.
 34. Apparatus asclaimed in claim 25, wherein in a mode of operation ions are urged alonga spiral or helical path through said apparatus.
 35. Apparatus asclaimed in claim 25, wherein ions are arranged to enter said apparatusvia said annular volume or via an aperture in said inner cylinder or viaan aperture in said outer cylinder.
 36. Apparatus as claimed in claim25, wherein ions are separated temporally according to their ionmobility or according to their rate of change of ion mobility withelectric field strength.
 37. Apparatus as claimed in claim 1, furthercomprising: a plurality of segmented planar electrodes; and wherein saidsecond device is arranged and adapted to apply DC voltages to saidsegmented planar electrodes so that one or more diagonal or inclined DCvoltage barriers are translated along at least a portion of the lengthof said apparatus.
 38. Apparatus as claimed in claim 1, furthercomprising: a plurality of inner ring electrodes and a plurality ofouter ring electrodes, wherein said inner ring electrodes and said outerring electrodes define an annular volume through which ions aretransmitted in use, wherein said plurality of inner ring electrodes orsaid plurality of outer ring electrodes are radially segmented into aplurality of segmented electrodes.
 39. Apparatus as claimed in claim 1,wherein said first device and said second device are operated atsubstantially the same time so that the net effect is to extend the pathlength of ions passing through said apparatus.
 40. A mass spectrometercomprising apparatus as claimed in claim
 1. 41. A method of separatingions according to one or more physico-chemical properties, comprising:creating an ion channel in which ions are confined, wherein ions areconfined within said ion channel in a first plane or confinementdirection by DC voltages, DC potentials or electrostatic potentials andare confined within said ion channel in a second plane or confinementdirection by RF voltages, RF potentials or pseudo-potentials, whereinsaid second plane or confinement direction is substantially orthogonalto said first plane or confinement direction; causing said ions toseparate according to said one or more physico-chemical properties alongan axis of said ion channel or along said ion channel towards a firstend; moving said axis of said ion channel or said ion channel away fromsaid first end, wherein an effect of moving said axis of said ionchannel or said ion channel away from said first end is to increase aneffective path length along which ions separate according to said one ormore physico-chemical properties; wherein said ions are caused toseparate according to said one or more physico-chemical properties in afirst direction or along said axis of said ion channel or along said ionchannel with a velocity which is substantially dependent upon said oneor more physico-chemical properties; and wherein said ions are driven ina second direction with a velocity which is substantially independent ofsaid one or more physico-chemical properties.